Liquid Crystal (LC) lenses and other liquid crystal optical devices are known in the art. One geometry is a planar construction in which a liquid crystal is held in a cell between glass or plastic plates. An electrically variable gradient index (so called GRIN) lens can be formed by controlling the relative orientation of the liquid crystal molecules to create a spatial variation of the index of refraction of the liquid crystal material within an aperture of the device. In this way, good optical lens power can be achieved within a relatively small thickness.
A variety of liquid crystal lens designs have been proposed that control the orientation of the liquid crystal molecules in response to an electric field. Most LC lens designs spatially modulate an electric field acting on the liquid crystal layer to create a resulting GRIN lens. In this area, a few approaches have been taken. Using relatively large voltages, it has been shown that a ring electrode placed at a distance above a liquid crystal cell under which a planar electrode is located can provide a GRIN lens. In an article published by A. F. Naumov et al., entitled “Liquid-Crystal Adaptive Lenses with Modal Control”, OPTICS LETTERS/Vol. 23, No. 13/Jul. 1, 1998, a lens such as that shown in FIG. 1 uses an LC layer 10 positioned between a hole patterned electrode 14 located adjacent to a top glass substrate 11, and a planar, optically transparent, electrode 12 of indium tin oxide located adjacent to a bottom glass substrate 16. Liquid crystal alignment layers 18 are located to either side of the LC layer 10. Because this prior art lens uses a single LC layer 10, the lens will be polarization dependent.
The principle of operation of the lens of FIG. 1 is the attenuation of the electrical potential, and corresponding drop in electric field strength across (in) the LC layer, between the periphery of the lens, where the hole patterned electrode 14 is located, and the center of the lens. Since the typical thickness of an LC layer 10 is about 0.05 mm, and the typical optical apertures of interest are about 2 mm, i.e. forty times larger, the radial drop in electric field strength across the LC layer 10 is drastic. For this reason, a high resistivity (or weakly conductive) layer 19 is deposited in the central part of the hole patterned electrode 14. The high resistivity layer 19 “softens” the drop in electric field according to the attenuation of electrical signals by the distributed RC circuitry formed by the high resistivity layer 19 and the rest of the system (where the resistance is provided mainly by the high resistivity layer 19 and the capacitance is provided mainly by the LC layer 10).
The GRIN lens of FIG. 1 is known to have some good properties, but suffers from some significant drawbacks. In particular, the operation of the lens is extremely sensitive to the geometrical and material parameters of the layered structure. The most important of these is the sheet resistance Rs of the high resistivity layer 19, which is defined by R=(dσ)−1, where d is the thickness of the high resistivity layer 19 and σ is its conductivity. This complicates greatly the fabrication of a polarization independent tunable liquid crystal lens (TLCL) based on this technology:
A liquid crystal lens layer will focus a single polarization of light and leave the other polarization essentially unaffected: the liquid crystal being a birefringent material, the light leaving the LC lens is structured into two polarizations. Natural light (obtained from sun or a lamp) contains a chaotic mixture of polarizations, and it is therefore desirable to use at least two liquid crystal layers each of which acts on a different (orthogonal) polarization direction so that all the light (all polarizations) is focused in the same way. A conventional approach uses a simple combination of two LC lenses, each having molecular orientations in mutually orthogonal planes. Thus, two planar liquid crystal lenses, each acting on a different polarization, are arranged with the intention that they will focus light onto a common focal plane. In practice however, the ability to create different “polarization” LC lenses having identical optical properties with respect to the image sensor is a challenge. A lens design that is too thick, with a large spacing between two liquid crystal layers, results in a large spacing between focal planes of different polarization components, and fails to create a clear image in natural light, due to each polarization component being focused in different way. In addition, when the lens shape and/or optical power of the two lenses are not identical, the effect of each lens is different even if the LC layers are positioned relative close to each other. This difference may arise because of differences in LC thicknesses or the sheet resistance values for two layers 19 of lenses which must be combined to allow polarization independent operation. While the thickness of the LC layers may be somewhat controlled by spacers, control of the sheet resistance is a much more difficult task (FIG. 3):
In wafer-scale manufacturing, a wafer is produced containing a large number of LC cells, and two such wafers are bonded together to make polarization independent LC optical devices. However, for such wafer fabricated lenses to have an identical optical power and lens shape (when the two wafers are bonded to each other), the two wafers must have the same properties.
In the case of a ring electrode which uses a highly resistive layer 19 of material placed near the aperture, the electrical sheet resistance Rs of the material plays an important role in defining the electrode and lensing properties. Controlling the resistance of a thin layer of material on a wafer within a required range for lensing operation is a challenge, and those resistive properties are very important to frequency control of the electrode.
One solution is proposed in PCT Patent Application WO/2009/153764 which describes two orthogonally oriented liquid crystal layers arranged, respectively, above and below a common, middle ring electrode, which is coated by a single high resistivity material used to control both LC layers. The single middle electrode is intended to provide a spatially modulated electric field for both the upper LC layer and the lower LC layer with each of the two layers acting on a different polarization direction of light. It is proven that, in this case, the two lenses image natural light in a substantially similar way onto the same imaging plane or image sensor. The spatial profile of the electric field (and thus the optical power) was shown to be the same for both the upper and lower layers. In manufacturing, the lower LC layer has the middle electrode placed on top of it (FIG. 2), and the upper LC layer is either fabricated on top of the middle electrode or separately fabricated and then bonded to the lower LC layer/middle electrode combination. When wafer-scale manufacturing a middle electrode arrangement, a small difference in position between the middle electrode and each LC cell can create a large discrepancy in the optical properties of each lens.